Currently, disc brake systems used in many passenger cars and the like are such that a pad is pressed against a disc rotor by making use of fluid pressure that is generated by a master cylinder or the like which is made to interlock with a brake pedal so as to apply the brake. In particular, in recent years, boosters have been put into practical use in which the braking force is increased by making use of negative pressure, compressed air, fluid pressure and the like in addition to human power.
In the disc brake systems which make use of fluid pressure and the like, however, piping is necessary for fluid, which needs, in turn, designs for avoiding the interference with other components and calls for a cost increase. Furthermore, in recent years, in association with the spread of ABS (antilock brake system) and TRC (traction control system), brake systems have been in demand which are quicker to respond and which can be controlled more accurately. This background has developed electric brake systems in which the driving force of an electric motor is used to thrust a piston to press a pad against a disc rotor (refer to Japanese Patent Unexamined Publication JP-A-3-41233).
Incidentally, in the electric brake systems, there may be a case where a ball screw mechanism or the like is adopted to convert the rotational force of the electric motor into a thrust by a piston (refer to Gazette of International Unexamined Patent Publication No. 99/60285, JP-A-10-257716).
Here, for example, in the electric brake system, in order to secure a sufficient braking force, an extremely large force needs to be secured for pressing the pad against the disc rotor. In contrast to this, the torque of an electric motor that can be installed on a vehicle is generally small. On the other hand, in the ball screw mechanism, when the lead angle of the screw is changed, the reduction ratio can be increased to some extent, but this generates problems. Then, in a system disclosed in the pamphlet of International Unexamined Patent Publication No. 99/60285, a large reduction ratio is given to the output of the electric motor using a bevel gear and a planetary gear mechanism, so that a large thrust is transmitted to the pad. When the transmission system that uses the bevel gear and planetary gear mechanism, there is caused a problem that the system becomes bulky as a whole and hence, a compact construction cannot be provided. In addition, when attempting to use a smaller electric motor, the reduction ratio needs to be increased further, this causing a problem that the transmission system becomes larger in size. In addition, in the system disclosed in JP-A-10-257716, a construction is described in which a single motor is used to rotate a screw shaft and a nut, and a spline groove is cut in the screw shaft, whereby the screw shaft is rotated by rotating a nut on the spline. The invention is such that the ball screw nut and the spline nut are driven by the single motor, while normally, two motors are used to rotate separately the ball screw nut and the spline nut in order to obtain two degrees of freedom in rotation and axial movement (for example, Japanese Patent Examined Publication 2722345). To make this happen, two sets of brakes are incorporated in a speed reducer so as to create a mode for rotating only the nut of the ball screw and a mode for rotating both the nuts. This requires a mechanism which uses a number of components and which is expensive and a sophisticated control. Since an actuator of the brake system does not need to rotate, an inexpensive and compact construction is desired which lacks a control for rotational directions.
The present invention was made in view of the problems inherent in the conventional techniques, and an object thereof is to provide an actuator which can exhibit a strong driving force while being kept compact in size and a brake system using the same.